Types of tracts

a) Ascending Tracts: If the neurons are present in lower part of CNS and their axons move upward, the bundles of these axons moving upward are called ascending tracts. These tracts are responsible to take information from lower part of CNS to the upper part.

b) Descending Tracts: If some neurons are present in upper part of CNS and their axons move downward, the bundles of these axons moving downward are called descending tracts. These tracts take information from the upper part of CNS to the lower part.

How Central Nervous System (CNS) does work?

When we touch an object, touch stimulus (mechanical stimulus) should be converted into electrochemical action potential. Because there is an apparatus at the site of touch which should convert stimulus energy into action potential, so that electrochemical fluctuation should move along the axons to CNS. This apparatus is called Receptor.

Receptors: Receptors are biological transducers, which are capable to convert one type of energy into another type. There are many types of receptors; some of them are discussed below:

Slight mechanical deformation in the tissue activates certain receptors. These receptors convert mechanical energy into action potential and it moves in the sensory nerve that eventually reaches to spinal cord (lower level of CNS). The information from the CNS lower level is taken to the higher level (cerebral cortex) through a chain of neurons. The bundles which take this information from lower level of CNS to the higher level are called ascending tracts or sensory system. If stimulus is unpleasant, cerebral cortex decides to pull the hand or fingers backward. This decision is carried from cerebral cortex to the lower level (spinal cord) by different bundles. The neuron carries this decision from the spinal cord to the effector (muscles) to pull the hand or fingers backward. These bundles carrying the information from the CNS higher level to the lower level are called descending tracts or motor system.

About spinal cord

Grey matter of spinal cord makes butterfly shape having ventral root and dorsal roots. Motor fibers come out CNS through ventral root (motor root) and sensory fibers enter to CNS through dorsal root. In dorsal root ganglion, there is a cell body of a neuron having a central process that goes to spinal cord and peripheral process. The information from periphery is taken to the dorsal root by peripheral process.

Dorsal root and ventral root unite together to form trunk of spinal nerve that is further divided into ventral ramus and dorsal ramus, usually innervating anterolateral parts of the trunk and the structure of back of the body respectively.

Anterior root (ventral root) is a pure motor root whereas dorsal root is a pure sensory root. Trunk and rami have both types of fibers i.e., sensory and motor fibers.

Receptors stimulating the sensory neurons take information to the spinal cord through dorsal root ganglion, and then ascending tracts take this information from spinal cord to the CNS higher level.

White matter of spinal cord is divided into:

1.Dorsal columns (dorsal column tract/system): Only ascending tracts pass through dorsal column. This is a modern and highly myelinated system with high velocity. There are following sensations which are carried by this system.

a) Fine touch.

b) Proprioception. (sensation coming from locomotor system)

c) Vibration.

d) Two points discrimination.

2.Anterolateral columns (anterolateral tract/system): Both ascending and descending tracts are passing through this system. This is a primitive and poorly myelinated system with low velocity. There are following sensations which are carried by this system.

a) Crude touch.

b) Temperature.

c) Pain.

d) Tickling sensation.

e) Itching

f)Sexual sensation.

Dorsal column tract/system

Information is taken from the receptors (meissner receptors, pacinian receptors, golgi tendon organ and muscle spindle) to dorsal root ganglion by peripheral process of the first order neuron. The cells body of first order neuron is present in the dorsal root ganglion. The central processes of this neuron enter to CNS and divides into a lateral root and medial root. Its medial root fibers go medially to dorsal column and move upward.

Sensation fibers from the lower part of the body (from lower limb upto T-6) are located medially in the spinal cord, making a special bundle that goes upward. This medial bundle is called fasciculus gracilis.

Sensation fibers from the upper part of the body are located laterally in the spinal cord, also making a special bundle that moves upward. This lateral bundle is called fasciculus cuneatus.

Clinical importance: If a patient has tumor in the medial part of the spinal cord then he/she will loose the dorsal column sensations in the lower part of the body and if a patient has tumor in the lateral part of the spinal cord then he/she will loose the dorsal column sensations in the upper part of the body.

First order neurons of dorsal column

Those are the neurons having their cell bodies in dorsal root ganglion. In dorsal column, axons from the lower extremity ascend as fasciculus gracilis; and axons from the upper extremity ascend as fasciculus cuneatus. These axons terminate by giving connections in nucleus gracilis and cuneatus respectively in the medulla.

Second order neurons of dorsal column

Second order neurons of dorsal column have their cell bodies in nucleus gracilis and nucleus cnueatus. Axons of those neurons emerge from respective nucleuses, cross to opposite side and move upwards making a very special type of another ascending system.

Internal arcuate fibers

The axons of second order neurons of dorsal column which decussate to contra lateral sides within the medulla are called internal arcuate fibers.

Medial lemniscus

Medial lemniscus is the the compact group of fibers which ascend after decussating. Left medial lemniscus contains information from right side of the body and right medial lemniscus contains information from left side of the body. Medial lemniscus terminates in VPL nucleus of thalamus.

Dorsal column system has major decussation in lower part of medulla. Then medial lemniscus system will pass through upper medulla, pons and mid brain.

Fasciculus vs. lemniscus

According to nomenclature, if lots of axon makes a circular bundle, this is called fasciculus. If lots of axon makes a flattened bundle, this is called lemniscus.

Thalamus

Thalamus is a very large piece of grey matter. It has special Y shaped white matter which divides thalamus into anterior, medial and lateral parts. Lateral part of thalamus is further divided into ventral part and dorsal part. Ventral part has three nucleuses which are ventroanterior, ventrointermediate and ventroposterior. Ventroposterior nucleus is further divided into ventropostero lateral (VPL) and ventropostero medial (VPM). VPL receive most of the sensation (touch, pain, vibration, position) from the most of the body. VPM receive sensation fibers from head and neck region.

VPL

This is the nucleus where second order neurons of dorsal column medial lemniscus system terminate and third order neuron starts.

Internal capsule

Internal capsule is a lot of pressed white matter between thalamus and caudate nucleus medially and lentiform nucleus laterally. Internal capsule is located in inferomedial part of the each cerebral hemisphere of brain.

Third order neuron of dorsal column

Anterolateral system

Group of ascending tracts which are located in anterior column and lateral column are called anterolateral system.

Pain and temperature pathway

In peripheral tissue, there are receptors for thermal stimulus and free nerve endings for pain. Thermal and pain sensations pathway are the same. There are two types of fibers in pain pathway to carry out information from periphery to CNS as following:

a) A- : These are lightly myelinated fibers. They carry fast pain.

b) C- fibers: These are non myelinated fibers. They are slow fibers.

Fast pain Vs. Slow pain

Fast pain e.g. pin prick pain travel through A- fibers, perceived by CNS in 0.1 seconds of the onset of pain stimulus. Fast pain is usually from the skin, stimulated by mechanical or thermal damages to the body.

Slow pain e.g. dull pain, burning pain, throbbing pain etc travel through C-fibers, perceived by CNS after one second or longer delay. Slow pain is usually from the skin or deeper tissue (viscera). Slow pain is stimulated by mechanical, thermal or chemical damages to the body. Slow pain fibers make multiple connections in grey horn, so their location is confused.

Pain is produced by tissue injury which leads to release of certain chemicals. These chemicals may be 5-hydroxytryptamine, histamine, bradykinin, acids and potassium. They stimulate the pain receptors.

Prostaglandins and substance P are the substances that reduce the threshold for the pain.

First order neuron of pain pathway

They have their cell bodies in dorsal root ganglion. Their fibers enter into spinal cord and terminate into substantia gelatinosa in dorsal grey horn.

Dorsolateral tract of Lissauer

As first order neuron enters into spinal cord, they give ascending and descending branches which contribute in the formation of local tract in the spinal cord. This tract is called dorsoalteral tract of Lissauer. These ascending and descending branches connect with second order neuron.

Second order neuron of pain pathway

Cell body of second order neuron is present in substantia gelatinosa of dorsal horn. Their axons decussate in the ventral white commissure and ascend in the lateral white column of opposite side. These ascending fibers terminate in the VPL nucleus of thalamus. This tract which is located in lateral part of spinal cord and goes up to thalamus is called lateral spinothalamic tract.

Pathway for crude touch

First order neuron of crude touch pathway has their cell body in dorsal root ganglion. Upon their entry into spinal cord, they also give ascending and descending branches. Their second order neuron decussate, go to anterior white column of contra lateral side and ascends toward thalamus. They form anterior spinothalamic tract.

Spinal lemniscus

Lateral spinothalamic tract, anterior spinothalamic tract and spinotectal tract fuse together at the level of medulla and ascend as a single bundle. This bundle is called spinal lemniscus. Spinotectal tract become the part of spinal lemniscus and terminates in tectum of midbrain.

Fast pain fibers terminate in VPL nucleus of thalamus. Slow pain fibers have different faiths. Some of them terminate in VPL nucleus; some fibers terminate in intralaminar nuclei of thalamus. On their pathway slow pain fibers also stimulate reticular formation.

Reticular formation

Reticular formation is mixture of grey and white matters extend throughout the brain stem. All the day reticular formation work and stimulate cerebral cortex. It play important role in maintaining consciousness. Reticular formation has connections with intralaminar nuclei of thalamus that stimulates whole cerebral cortex. Reticular formation keeps whole cerebral cortex active via intralaminar nuclei. All ascending tracts give collaterals to reticular formation. Slow pain fibers make special connections with reticular formation.

Third order neuron of anterolateral tract system

These neurons have their cell bodies in VPL and their axons pass through posterior limb of internal capsule, and their final destination is post central gyrus (sensory cortex).

Some fibers of pain pathway will make special connections to cingulate gyrus that deals with associated emotional component of pain and there are some other fibers which make connections to insular cortex that initiate response to associated autonomic component of pain.

Gate control theory

It concerns with any stimulus that activates the connector neuron which inhibit the pain sensation to reach CNS may lead to suppression of pain.

Descending analgesia system

System of descending fibers from periventricular grey matter, periaqueductal grey matter and many mid line nuclei (e.g. nucleus raphe) which release some morphine like (enkephalins, endorphins) neurotransmitters in posterior horn of spinal column and inhibit the pain transmission.

Key points while learning pain

a) Pain transmission

b) Pain perception

c) Pain modulation

Dorsal spinocerebellar tract

These are the fibers which take the information of locomotor system from spinal cord up to the cerebellum to plan the movements and postures appropriately. First order neuron has their cell bodies in dorsal root ganglion and terminates in CLARKE nucleus in dorsal grey horn. Second order neuron ascends through ipsilateral white lateral column and connects with cerebellar cortex of same side via inferior cerbellar peduncle.

Clarke nucleus

This is a large column of neurons which is located in the dorsal grey horn of the spinal cord, extending from C8 to L3. It receives information from lower limbs and trunk.

Cerebellar peduncles

These are specialized bundles of white matter which connect brainstem with cerebellum.

Accessory cuneate nucleus

Spinocerebellar tract fiber, above the C8 level enters into the nucleus then ascends and pass through inferior cerebellar peduncle.

Cuneocerebellar tract

The first order neuron is found in the dorsal root ganglion from C2 to T7 and project to accessory cuneate nucleus. The second order neuron connects cuneate nucleus with ipsilateral cerebellum after passing through inferior peduncle. These are the counterpart of dorsal spinocerebellar pathway from the upper limb.

Anterior/Ventral spinocerebellar tract

First order neuron of this tract has their cell body in dorsal root ganglion and terminates in spinal cord. Axons of second order neuron decussate at the same level in the ventral white commissure and ascends forming anterior spinocerebellar tract. These fibers enter into cerebellum through superior cerebellar peduncle; then cross again and terminate into the contra lateral cerebellar vermis. In this way, information from right side reaches to ipsilateral cerebellar cortex.

Spinotectal pathway

This is the pathway in which cell bodies of first order neuron is present in dorsal root ganglion and connects with second order neuron at the level of entry. Axon of second order neuron decussates and ascends in contra lateral white column as spinotectal tract to terminate in tectum in the dorsal part of the mid brain. In the medulla, it joins the lateral spinothalamic tract and anterior spinothalamic tract to become part of spinal leminiscus. This pathway controls the spinovisual responses.

Spino-olivary pathway

In this pathway, cell body of first order neuron present in the dorsal root ganglion. Cell body of second order neuron is present in grey horn in spinal cord; its axon decussates and ascends in contra lateral white column as spino-olivary tract to terminate in olive in the medulla. From olive next order neuron connect with cerebellum via inferior peduncle. This is the next pathway for the proprioception, touch, pressure and sense of position.

Spinoreticular pathway

There is another pathway in which fibers enter into spinal cord and ascend through ipsilateral white column as spinoreticualr tract and terminate in the reticular formation. These fibers simply stimulate the reticular formation to increase the whole conscious level.

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